The control and monitoring of gas flow is important in many applications, from fluid transport and delivery, to medical applications such as breath analysis, for anesthesiology and diagnostic monitoring.
Volatile organic compounds (VOCs) are a diverse group of carbon-based chemicals that are volatile at ambient temperature. VOC analysis is used in the fields of environmental contamination monitoring, forensic science, mining, chemical manufacture and process control, and fragrance and flavor industries. VOCs emitted from breath may include biomarkers useful in the diagnosis of a plurality of diseases, including but not limited to asthma, breast cancer, chronic obstructive pulmonary disease, diabetes, schizophrenia, cystic fibrosis, and arthritis. For example Nitric Oxide (NO), Pentane, ethane, and 8-isoprotane have been found as potential diagnostic markers in human breath of patients with asthma, while acetone, ethanol, and methyl nitrate have been found in diabetes. Other VOCs that could be used as biomarkers in breath include, but are not limited to carbon dioxide, oxygen, and ammonia.
In general, biomarker monitoring offers a lower cost, less invasive method of obtaining important medical information from a patient. Since the discovery of NO in human breath, it has been quite intensively studied with association in allergen-induced airway inflammation, as a marker of eosinophilic airway inflammation, and its utility to assess response to anti-inflammatory therapy. NO levels are affected by age, height, weight, gender, and race and NO is generally free of day-to-day variation making it highly reproducible. Also, NO concentrations in exhaled breath are dependent on the flow rate making NO the most widely studied and standardized noninvasive biomarker for asthma.
Biomarker analysis through exhaled-breath monitoring has not yet been introduced into every day diagnostic clinical practice. This is mainly due to the lack of standardization and normalization of sampling procedures and the lack of a uniformly accepted evaluation criteria for data. However, with the recent development of flow rate sensors, measurement of biomarkers such as NO has been greatly simplified.
The alternative to assessing airway inflammation is bronchial biopsy which is expensive, invasive, complex, and not widely available to physicians. Bronchial biopsy does not fit the trend of moving towards non-invasive and rapid diagnostic tools which are simpler to perform, painless, and more agreeable to patients. This new trend opens up a new promising area for a noninvasive diagnostic tool with various advantages. Tests can be repeatedly performed without discomfort to patients. They also can be applied to children including infants and to patients with severe disease in whom more invasive procedures are not possible.
Existing methods and devices may detect such unknown biomarkers, but they are generally slow and complicated. Miniaturized sensors and methods generally lack sufficient sensitivity, selectivity, and/or reliability; and may be especially deficient for detecting one or more biomarkers in human breath.
Even though flow sensors can be quite affordable, a simpler, less expensive and more reliable sensor can be tremendously useful, particularly in applications targeting in-home medical treatment/management tools. Other flow sensors in the art tend to use complicated detection methods such as thermal mass transport based, optical based, mechanical (both flow and pressure based), Doppler, electromagnetic, and Coriolis effects. One of the more common techniques to measure flow rate is using an ultrasound transducer.
U.S. Pat. No. 4,603,589 uses two ultrasound bi-directional transducers, each include transmitting and receiving circuits, to measure the flow parameters between the two transducers. The international patent application WO 2007/065476 also uses two ultrasound bi-directional transducers but instead calculates a mass flow of gas by obtaining time of flight signals for the upstream and downstream. U.S. Pat. No. 2002/0062681 uses two ultrasound transducers, one near the gas inlet port to a gas flow path and one near the outlet of the gas outlet port. A microcontroller compares the difference in time between the ultrasonic pulse from the ultrasound transducer near the inlet and the ultrasound transducer near the outlet and measures the concentration of gas flowing through the device.
The above prior art systems incorporate ultrasound or time and flight or a combination of both to calculate gas concentration or flow rates. These methods involve complicated and expensive circuitry and electronics.
Thus, there is a need in the art for a simple and inexpensive technology for measuring air flow, one that can be reliably integrated with a sensor system for biomarker monitoring.